Chromium stainless steels containing chromium as a main alloying element are classified into martensitic and ferritic stainless steels. Compared with austenitic stainless steel containing a relatively high amount of nickel, they are inexpensive and feature such properties as ferromagnetism and a low coefficient of thermal expansion. There are therefore many applications in which chromium stainless steels are used not only for economical reasons but also for their properties.
Particularly in the field of electronic instruments and precision machine parts where such chromium stainless steels are used, along with the increasing demand of recent years, the requirements for steel sheet materials are becoming more rigorous. Steel sheet materials are required that possess combinations of properties that may be in conflict, such as for example high strength and high elongation, and good shape and thickness precision before working together with good shape precision after working.
Conventional chromium stainless steels having high strength include martensitic stainless steels. For example, seven types of martensitic stainless steel are prescribed in the cold rolled stainless steel sheets and strips of JIS G 4305. The prescribed carbon content of these martensitic stainless steels ranges from up to 0.08% (for SUS410S) to 0.60-0.75% (for SUS440A), a high C content compared with ferritic stainless steels of the same Cr level. High strength can be imparted to these steels by quenching treatment or by quenching and tempering treatment. As indicated by the name, the structure of martensitic stainless sheets subjected to such heat treatment is basically martensitic. While this gives the steel great strength (hardness), elongation is extremely poor.
Accordingly, as martensitic steel that has been quenched (or quenched and tempered) has poor workability, steel manufacturers usually ship the material in the annealed state, that is, as soft ferritic steel sheet or strip having low strength and hardness, to a processor where the material is worked into product shape and is then subjected to quenching or quenching and tempering treatment.
In many cases surface oxide film or scale formed by the post-forming heat treatment is undesirable with stainless steel in which the emphasis is on surface attractiveness. As a countermeasure, it therefore becomes necessary to carry out the heat treatment in a vacuum or in an inert gas atmosphere, and to pickle and/or polish the steel after the heat treatment. Thus, using martensitic steel has tended to increase the burden on the side of the processor, unavoidably increasing the cost of the final product.
On the other hand, ferritic stainless steel has never been used much in applications requiring high strength, and hardening by heat treatment has not been much expected. In some cases annealing is followed by work hardening using temper rolling (cold rolling) to obtain ferritic stainless steel having high strength. In this case the steel is used in the cold rolled state, and a problem is that while increasing the rolling reduction rate increases the strength, above a certain point the result is a marked degradation in the elongation, meaning there is an upper limit to the level of strength at which a certain degree of workability can be maintained.
The properties of SUS430 strengthened by cold rolling at 20-30%, for example, show a poor strength-elongation balance, with a hardness of around HV 230 and no more than 2 or 3% elongation. Moreover, using temper rolling to obtain wide material formed to a good shape is itself difficult, and the material exhibits considerable plane anisotropy regarding strength and elongation, making it difficult to obtain good shape precision after working.
To solve the above problems of conventional high strength chromium stainless steels, the present inventors have proposed, for example in JP-A-63-7338, JP-A-63-169330 to JP-A-63-169335, JP-A-1-172524 and JP-A-1-172525, a process for the production of a strip of a chromium stainless steel of a duplex structure consisting essentially of ferrite and martensite and having high strength and elongation, which process comprises the steps of basically hot rolling and cold rolling a slab of a steel to provide steel strip, said steel having a composition adjusted to form a structure of ferrite and austenite at high temperature, continuous finish heat treatment in which the steel strip is heated to an appropriate temperature above the Ac.sub.1 point of the steel to form a two-phase of ferrite and austenite and maintained at that temperature, and the heated strip is cooled at an appropriate cooling rate to transform the austenite to martensite.